As Scribbler said, you are way too invested in this. Almost oddly so.
It’s cool that you donated a gazebo or two. You donated them. You gave them. No one owes you anything and while you may have improved the stable atmosphere with these gifts, that does not put you above anyone else. I, personally, don’t have much time to sit around at the stable, so this would not matter to me. Others may feel the same way, so if they are not used so much or mentioned, it’s not personal. If they are used, that’s great, but again does not make you above anyone else at that stable because you chose to gift something.
If you are really hating life at this stable, 7 miles to the other stable is not that far? I don’t understand that part. Unless you live in a horribly congested area and those 7 miles are going to take 30+ minutes.
I understand that this is your hobby/passion/downtime/investment/whatever so it can really put a thorn in your side to have someone disturb that. We know that we will not be liked by everyone, and vice versa, but it’s especially difficult when that blends into a hobby that you are passionate about. That being said, don’t focus on this individual and what they are doing or what they are going to be doing. I think that you went a bit far when you speculated about their future children. That’s a bit much.
Who cares if she trains horses for little income? Who cares what she rides? As long as she isn’t riding your horse, that’s none of your business and between those parties actually involved.
I understand you want some leverage with the 6 month board thing, but it’s also weird to put your barn owner in that position. Somewhat manipulative. If this individual is putting people in harms way, is a safety concern, or damaging the environment (not just yours) that badly, it warrants an adult discussion with the barn owners. If they give you feedback you don’t like, either swallow it or leave.
Perhaps there is more to the story of this individual. Maybe she has had really horrible life circumstances outside of the stable and the barn owners take pity and allow this behavior. I am not saying that is right, but they could be in an awkward spot too. Or maybe they do need people to voice (mature!) concerns as others have just skated around the matter in the past and they don’t know how much it is impacting the stable.
You do what you can do. You ignore her, you don’t think about her, her life, her future plans, her training, her income, her whatever! You calmly and matter of fact discuss with the barn owners. You go about your business. Do not let it get to you. If things get too bad and you do not like the barn owners management of it, you are free to leave and drive that whopping 7 miles.
I will admit that this is a crap time of year for equestrians. The cold, the weather, the lack of daylight - - it’s all getting better though! So sometimes things irritate us more than usual and things seem more bleek than they are. So maybe that plays into it? I’d focus on you and your trainer. Do not worry about what ifs with this person. If she does video something, handle it then. All the worrying in the world isn’t going to stop her. You should be able to notice if she is making a video and stop her directly right then and there. If she has never done this before, what makes you think she’d choose you? Relax. Ride.